The present invention relates generally to an orthopedic implant assembly system and in particular to a multi-axial bone plate system.
Orthopedic implant assembly systems having a multi-axial bone plate are known in the art and include at least two pedicular screws anchored in adjacent vertebrae to be treated and a connecting plate designed to connect the screw heads together in a rigid manner. Previous references, such as U.S. Pat. Nos. 6,287,309 and 5,486,176 disclose a system of this kind in which each pedicular screw has a hexagonal section part for inserting the screw into the bone and on top of which is a threaded shank. The plate has a plurality of oblong openings through which the various threaded shanks can be inserted and an open groove on its bottom face to prevent rotation of the hexagonal part of each pedicular screw.
Each pedicular screw is associated with a stirrup through which the threaded shank of the screw also passes and which straddles the top of the plate. Finally, a nut is screwed onto the threaded shank to trap and immobilize the plate with the stirrup on top of it, between it and the hexagonal portion of the screw.
Further, known systems include at least two pedicular screws and a connecting plate for linking screws together in essentially a rigid manner. Each screw has a bone anchor threaded part, a non-circular section head, and a threaded end shank adapted to cooperate with a nut. The plate has at least one opening adapted to have the threaded end shank of the screw pass through it and be trapped between the screw head and the nut. Raised patterns are provided on the top face of the plate and on the bottom face of the stirrup to prevent longitudinal sliding of the plate relative to the screw. A locking member is also provided for preventing relative angular movement between the heads of the screws and the connecting plate. The locking member is adapted to be inserted between the plate and the screw head and includes a bar through which the threaded end shank of the screw passes. The locking member further includes a first locking cooperation of shapes with the screw head and a second cooperation of shapes with the plate.
Although these systems are generally satisfactory, they nevertheless have certain drawbacks. Specifically, in certain systems, raised patterns have to be provided to prevent sliding because the plate and the stirrup cooperate via two plane faces in compression. Absence of the raised patterns would lead to the risk of entirely unacceptable relative movement of the vertebrae. The machining required to create these raised patterns significantly increases the mean cost of the plates and the stirrups.
Additionally, the raised patterns can impede fine adjustment of the system. For example, there are only a particular number of discrete mutually engageable positions of the plate and the stirrup, i.e., a particular number of discrete distances between the screws. Moreover, if the nut is over-tightened before the final tightening, lateral sliding of the plate and the stirrup during adjustment may be impeded.
Some of the drawbacks associated with other designs include that the first locking cooperation and second cooperation of shapes with the plate forbid and restrict plate movement in an anterior and posterior direction once the pedicle screw has been rested against the plate member.
Additionally, most plate systems include a bone fastener with a threaded end extended from the vertebral body. In order to lock the screw relative to the plate, a nut must be used in combination with the screw. The problem associated with this design is that if the screw is not placed at the correct depth within the vertebral body, the assembly must be disassembled in order to either increase or decrease the depth of the screw in the vertebrae. An additional problem results from this action due to the fact that once the screw has been placed too deep within the vertebral body, the screw might be less securely locked within the vertebral body when the screw is backed out and placed in its correct position.
Other shortcomings of known systems include angled orientations in the sagittal direction are not permitted due to the shape of the locking cooperation members.